The present invention relates to cathode ray tubes having focus masks, and more particularly to a tube having a focus mask with a mask structure and support frame with an improved border.
A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate panel of the tube and is made up of an array of elements of three different color-emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask having strands, is located between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors to emit light so that an image is displayed on the viewing surface of the faceplate panel.
Another type of shadow mask is a focus mask having a mask structure comprising an array of parallel conductive strands that are tensioned onto a mask support frame and an array of parallel conductive cross wires. The strands terminate at top and bottom with an etched solid border that is welded to the support frame. The solid border of the mask serves as an optical edge for forming the black surround of the matrix which in turn defines the perimeter of the screen array of the tube screen. The cross wires are placed perpendicular to the strands and separated from the strands by an insulator. The two arrays of conductors form apertures, or mask openings, between the solid borders called the active array area. Different high voltages are applied to each array of conductors, thereby providing an electric magnifying lens, or focus lens, for accelerating and focusing the electron beam in each of the mask openings of the active array area.
The cross wires are typically applied to the screen side surface of the strands and overlie at least a section of the solid borders of the mask near the active array edges. It has been found that the solid borders of a focus mask tend to prevent proper distribution of the insulator and necessary insulating capabilities between the two arrays of conductors near the active array edges where the cross wires overlie the solid borders. Improper insulation of the strands from the cross wires may lead to arcing between the conductors at one or more places near the active array edges which may result in an electrical short leading to the subsequent failure of the focus mask.
Such problems with the mask solid borders have led to the removal of the cross wires near the solid border region or individually attaching the mask strands to the mask support frame, wherein no solid mask borders are attached to the frame. However, removing the cross wires near the solid border region eliminates focusing apertures along the active array edges which in turn reduces the focusing region of the tube. In addition, individual attachment of the mask strands has been problematic because the strands tend to displace from the pushing action of the weld device during welding. Furthermore, the absence of a solid border is also not desirable because the solid borders serve as optical edges at the top and bottom location of the tube, for proper matrix printing.
It is therefore desirable to provide an improved border for a focus mask for use in a cathode ray tube.
The present invention provides a focus mask and frame assembly for a cathode ray tube. The cathode ray tube includes an evacuated envelope enclosing a luminescent screen, a focus mask, and a support frame for holding the focus mask in register with the screen. The focus mask includes a plurality of parallel spaced-apart strands and a plurality of spaced-apart cross wires separated from the strands by an insulating material and oriented substantially perpendicular to the strands forming an active array area of focusing apertures. A novel border is attached to the strands opposite the insulating material and extends into the upper and lower outer edges of the active array area wherein the cross wires overlie a portion of the borders.